From EP 0 806 306 B1 it is known to provide in the wheel electronics a movement sensor, namely an acceleration sensor, which responds to radial accelerations and/or tangential accelerations occurring at the wheel. Such an acceleration sensor permits not only a differentiation between a resting vehicle and a stationary vehicle, but it also delivers information relating to the rotation speed of the wheel. The radial acceleration and the tangential acceleration of a wheel is superimposed by the gravitational acceleration, the influence of which changes its sign twice on each revolution of the wheel. The reciprocal value of the time span between every two sign changes of the superimposition of the acceleration signal by the gravitational acceleration is proportional to the rotation speed of the wheel.
The method known from EP 0 806 306 B1 compares the rotation speed obtained by the acceleration sensor in the wheel electronics with the rotation speeds which are delivered for the same period of time by ABS sensors on the wheels of the vehicle. If the coinciding of the rotation speed measurement originating from the wheel electronics with a rotation speed measurement originating from an ABS sensor is established, then it is assumed that the ABS sensor and the respective wheel electronics are assigned to the same wheel position, e.g. at the front left on the vehicle. A prerequisite for this, however, is that the rotation speed measurement originating from the wheel electronics coincides with the rotation speed measurement of only one of the ABS sensors. The identifier is than stored under the position of the ABS sensor from which the coinciding rotation speed signal came. In the case of future signals which contain the same identifier, the evaluation unit then knows from which wheel or respectively from which wheel position, e.g. front left, the signal originates.
The known assignment method presupposes that in the rotation speed comparison the wheels have differences in the rotation speed. These differences can have different causes, e.g. slippage between wheel and road surface, different rolling radii as a result of different tire sizes or as a result of different loads or as a result of different tire pressures and in particular as a result of cornering. Wheels on the outer side of a curve always cover a greater path than wheels on the inner side of a curve and therefore rotate more quickly than the wheels on the inner side of the curve. The differences in rotation speed between the wheels of a vehicle can be small, when travelling in a straight line they can even be vanishingly small. EP 0 806 306 B1 therefore requires that two rotation speed values, one of which has been measured by means of the acceleration sensor in a wheel electronics and one of which has been measured by means of an ABS sensor, are only evaluated as being coinciding when they coincide except for disparities which are unavoidable with respect to measurement technology. Only when such a high-grade coincidence is established a single time for only one of the ABS sensors is the identifier of the wheel electronics assigned precisely to the ABS sensor which has delivered the coinciding rotation speed signal. This has the disadvantage that it can take a very long time until the evaluation unit has assigned all the identifiers which it receives respectively to a particular ABS sensor and therefore to a particular wheel position. In addition, the known method is susceptible to erroneous assignments. However, as long as the identifiers of the wheel electronics installed on the vehicle are not correctly assigned to the wheel positions, a reliable tire pressure monitoring is not possible. The method known from EP 0 806 306 B1 has therefore not entered into practice.
From DE 198 56 861 B4 an automatic assignment method is known, which via the evaluation of rotation direction information which is delivered from an acceleration sensor in the respective wheel electronics, and from the received signal levels, enables a definite assignment of the identifiers to the wheel positions. However, this requires the arrangement of the receiving antenna at an eccentrically situated site of the body floor pan as a prerequisite that the signals which are transmitted from the wheel electronics with the same power are received by the receiving antenna with different signal levels. This possibility, which is good per se, for carrying out an assignment method rapidly and reliably, is out of the question, however, if a central position is required for the receiving antenna, at which it can receive signals from all directions in as equally good a manner as possible, e.g. also the signals of a wireless key, which can strike the receiving antenna from any direction.
From EP 0 861 159 B1 it is known to determine the assignment of the identifiers to the different wheel positions solely from the intensity of the signals received from one or more antennae through statistical evaluation of the measured intensities. The disadvantages indicated with regard to DE 19 856 861 B4 also apply to EP 0 861 159 B1. In addition, as a further disadvantage, is that the assignment method lasts all the longer, the fewer receiving antennae are connected to the receiver. With only one receiving antenna, the duration of the assignment method is the longest, and the possibilities for positioning the antenna are very limited, because the antenna must be able to receive signals from all the wheel electronics and the received intensities must differ from one another.
A method having the features of the preamble of claim 1 is known from DE 10 2011 050 636 B4. In this method, the evaluation unit detects the identifiers contained in the received signals, measures the time span between impact signals, which have been received consecutively, multiplies the measured time span with a speed of the vehicle measured in the same time span and reported to the evaluation unit, checks whether the length formed through this multiplication coincides, within a predetermined tolerance, with a given axle distance of the vehicle, assigns, in the event that it coincides, the identifier which was contained in the signal, which contains the information relating to the first of two consecutively detected impacts, to a front of the two axles and assigns the identifier which was contained in the signal, which contains the information relating to the second of the two consecutively detected impacts, to the rear of the two axles, when the signals are received during forward travel.
This method uses only a single antenna for receiving the signals. The determining of an antenna position which is suitable for the method is complicated and is frequently possible only shortly before the start of production of a vehicle series. Wheel electronics which transmit from a position which is unfavorable for the receiver antenna are particularly difficult to assign, their assignment can take a particularly long time.
In order to be able to clearly differentiate between signals which come from front wheels, and signals which come from rear wheels, it is expedient, in a two-axle vehicle, to arrange the receiving antenna close to the front axle or close to the rear axle of the vehicle. In order to be able to differentiate by means of the signal intensity between signals which come from wheels on the right side of the vehicle and signals which come from wheels on the left side of the vehicle, it is expedient to arrange the receiving antenna remote from the vertical longitudinal centre plane of the vehicle. The above-mentioned restrictive specifications for the location of the receiving antenna cannot always be complied with, or can only be imperfectly complied with. This disadvantage is further intensified in that the transmitters of the wheel electronics can have different transmission powers due to manufacturing tolerances, that the tires which are mounted on the vehicle can damp the transmitted signals to a different extent, and that the signals can be shielded to a different extent by components of the vehicle depending on the position of the rotating wheels.
Regardless of these difficulties, there are vehicle manufacturers who require that with each new start of the vehicle the complete assignment method is to run, and namely, for safety reasons, as quickly as possible and with as few transmission processes as possible, in order to shorten as little as possible the lifespan of the battery, arranged in the wheel electronics, through the frequent assignment methods, because the battery cannot normally be replaced.